This invention relates to a liquid crystal display and, more particularly, to a method and an apparatus for driving a passive liquid crystal display with an enhanced select/nonselect ratio.
Large passive liquid crystal displays suffer from serious contrast problems due to the manner in which the display elements are actuated. A typical passive liquid crystal display 10 is shown in simplified schematic form in the FIG. 1. A display panel 12 has a plurality of horizontally row electrodes or lines 14 extending perpendicularly to a plurality of column electrodes or lines 16. The crossing points of the lines 14 and 16 define pixels 18 for displaying visual information. For illustration purposes, the display is shown as having four row lines 14, R1 through R4, and five column lines 16, C1 through C5. In practice there are often hundreds of rows and columns.
Voltages are applied to the columns and the rows, and a pixel having both its row and column carrying a voltage is addressed. Liquid crystal display devices typically use what is generally referred to as "line at a time" addressing method, which is depicted by the wave forms shown in FIG. 2. All of the column lines 16 are activated simultaneously by the application of a column voltage wave form V.sub.C (t.sub.K) having a plurality of pulses. The row lines 14 are each turned on for a fixed period of time in sequence. Specifically, at a time t.sub.1, a first row voltage wave form V.sub.R1 (t.sub.K) includes a pulse which is applied to the row line R1, while an associated column voltage pulse is applied to column lines C1 through C5 such that all of the pixels 18 in the first row of the display panel 12 are activated. At a time t.sub.2, after the first row voltage pulse has been terminated, a second row voltage wave form V.sub.R2 (t.sub.K) applies a pulse to the row line R2 while an associated column voltage pulse is applied to the column lines C1 through C5 such that the second row of the pixels 18 in the display panel 12 is activated. Similarly, row voltage wave forms V.sub.R3 (t.sub.K) and V.sub.R4 (t.sub.K) apply pulses to the row lines R3 and R4 respectively; at the times t.sub.3 and t.sub.4, respectively, to activate the pixels 18 in the third and fourth rows, respectively. Once activated, the pixels remain at the same state for a limited period of time (referred to as the decay time).
In a large passive display, the pixels 18 located in the upper rows may have relaxed entirely before the lower rows are addressed such that the image in those upper row portions of the display will fade. This results in a low contrast ratio. The problem assumes critical importance when the device is run in a video mode, where short relaxation times are required.
One attempt to deal with this problem has been a method wherein all of the rows are driven simultaneously. The formulas for setting the voltages on the columns include a coefficient. In a prior art, a column coefficient was presetted, and plugged into the formulas used to calculate the required voltages. However, for images that have more than one bit of gray scale, this has resulted in a coupling between the different pixels in a given column. This coupling manifests itself such that the required rms (root mean square) voltage to achieve a particular light transmission intensity in a given pixel depends on the state of all of the other pixels in that column. Decoupling the various pixels within a column from one another requires the use of a virtual row.
In addition, with this approach, the so called Alt and Pleshko limit has been understood in prior art to place a theoretic maximum to the select/nonselect ratio. The select/nonselect ratio is the rms voltage range between a low limit and a high limit attainable with the driving signals. The liquid crystal material determines the on and off states of display corresponding to these voltage limits. It is between these limits that any gray scale, or variation in the intensity of a particular pixel, must be achieved. A designer of LCDs would like to maximize the select/nonselect ratio for several reasons. The video mode requires fast liquid crystal materials with short relaxation times to minimize "ghosting". The faster liquid crystal material in general has much less steep BV curve and hence the larger select/nonselect ratio is essential to have required high contrast ratio. Moreover, larger select/nonselect ratio allows for more accurate specification of gray and hence larger available number of gray levels.